I'm working on making some chimes, which involves tying a (tuned) piece of metal to a piece of wood fairly tightly - ideally, so that there is always a gentle force pulling the chime together with the wood. For context, here are two pictures of the setup, one from the top and one from the end: (Ignore the holes in the aluminum; they are from an earlier, less successful experiment.)
The string passes through the holes to the side of the tube, and loops around the tube itself on top and is tied underneath the wood. The knot needs to be tight because, if it is not, the tube can jump around when hit with a mallet (which is bad) and the vibrations of the tube can create buzzing against the wood (which is also bad). This design seems to yield really good results when I test it out while holding the strings taught prior to tying them, but it doesn't work very well when I tie the knot. (Also, since it's probably important: I've been using overhand knots to make the loops of string - it's the only knot I'm familiar with, so I would be satisfied with an answer that points towards a more appropriate knot to use, if that's the problem)
Is there some technique I can use to ensure that a loop of string stays under tension after I tie it into a knot?